Field of the Invention
The present invention relates to a touch panel device.
Description of the Background Art
Touch panels that detect touches of indicators, such as fingers, and specify coordinates (touch coordinates) indicating the positions of the touches have been widely used in various information processing devices. In particular, touch screens provided with touch panels on the screens of display devices have been popular as new user interfaces of the information processing devices.
Known methods of detecting the touch in the touch panel include a resistance film method and a capacitance method, for example. One example of the capacitance method is a projected capacitive touchscreen (PCT) method. A touch panel in this method is capable of detecting a touch of an indicator even if the front surface of the touch panel is covered by a protection plate (e.g., a glass plate) having a thickness of about several mm. Consequently, sturdiness can be achieved by using the protection plate. Moreover, such a touch panel, which is free from a moving part, has a long life. The PCT touch panels with such advantages have been widely used in apparatuses, such as portable communication equipment including smart phones, automated teller machines (ATMs) in financial institutions, and car navigation devices (Japanese Patent Application Laid-Open No. 2008-134836).
It is important in the touch panels not to receive user's unintended input operations (operating errors). Unfortunately, the PCT touch panels have been seriously facing malfunctions due to electrostatic discharge (ESD) (Japanese Patent Application Laid-Open No. 2013-161397). The ESD occurs because, for example, static electricity excessively charged in a user's body is applied to the touch panel via a user's finger.
In a typical PCT touch panel, a plurality of sensors within a touch sensor panel are electrically connected to a semiconductor integrated circuit of a complementary metal oxide semiconductor (CMOS) structure that detects a capacitance. The ESD applied to the touch sensor panel is input from an input terminal of the semiconductor integrated circuit. When the input ESD has a voltage that exceeds the rating of the semiconductor integrated circuit, i.e., a voltage higher than a power supply voltage or a voltage lower than a grounding potential (GND), a latch-up phenomenon in which a large current flows through an unintentional parasitic transistor may occur. In an abnormal state (a latch-up condition) having the latch-up phenomenon, the touch panel fails to function as it should be and further possibly results in its destruction. Moreover, a known method of restoring a latched-up semiconductor integrated circuit to an original normal state includes turning off the power supply of the semiconductor integrated circuit, thus to stop current flowing through the parasitic transistor before turning back on the power supply (Japanese Patent Application Laid-Open No. 59-149424).
Using the restoring method in Japanese Patent Application Laid-Open No. 59-149424 when the ESD renders the touch panel device abnormal loses information about capacitance values and coordinates just before the touch panel device becomes the abnormal state due to the turning-off of the power supply, where the information is stored inside the semiconductor integrated circuit that detects the capacitance of the touch sensor panel. This possibly has an effect on the operation of host equipment connected to the touch panel device, such as incorrect coordinate information is output or coordinate information necessary to indicate completion of a touch operation (touch up) is lost, when the power supply is turned back on.